1. Field of the Invention
The present invention relates to a controller for an internal-combustion engine capable of varying an open valve period and a lift amount of an intake valve.
2. Description of the Related Art
There has conventionally been known a controller for an internal-combustion engine which has been disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-180306 (hereinafter referred to as patent document 1) and which is provided with, as intake cams used for opening/closing an intake valve, a plurality of intake cams having different profiles that specify the angle width and the lift amount in an open valve period of the intake valve. The controller is constructed such that the phase angles of the intake cams relative to the phase angle of a crankshaft (the output shaft of the internal-combustion engine) can be changed within a predetermined angle range.
In this type of internal-combustion engine, the intake cam used to actually open/close the intake valve is selectively switched by a valve lift changing mechanism, such as VTEC (a registered trademark), to variably change the angle width and the lift amount of the intake valve in the open valve period.
Further, the phase angle of each of the intake cams relative to the phase angle of the crankshaft (the output shaft of the internal-combustion engine) is continuously changed by a valve phase changing mechanism, such as VTC, thereby to change the phase angle in the open valve period of an intake valve (a pair of the phase angle at a valve opening start and the phase angle at a valve opening end) driven by each of the intake cams.
For example, the controller for an internal-combustion engine disclosed in patent document 1 has three intake cams, switches among the intake cams, and changes the phase angle of each intake cam according to the load on the internal-combustion engine. In this case, the switching among the intake cams is implemented such that the angle width and the lift amount of the intake valve in the open valve period increase as the load on the internal-combustion engine increases. Further, when the intake valve is driven to open/close by each of the intake cams, the phase angle of each of the intake cams is changed such that the phase angle at the start of the opening of the intake valve approaches the phase angle at a top dead center (TDC) from a phase angle on a lead angle side relative to the phase angle at the top dead center as the load on the internal-combustion engine increases.
In the controller for an internal-combustion engine disclosed in patent document 1, when switching among the intake cams, which open/close the intake valve, is implemented, the phase angle of an intake cam on a low load side, at which the angle width and the lift amount of the intake valve in the open valve period are small, is controlled to a phase angle that causes the phase angle at the opening start of the intake valve defined thereby to substantially coincide with the phase angle at the top dead center (TDC). Further, the phase angle of an intake cam on a high load side, at which the angle width and the lift amount of the intake valve in the open valve period are large, is controlled to a phase angle that causes the phase angle at the opening start of the intake valve defined thereby to advance to become larger than the phase angle at the top dead center (TDC).
In this case, an unsmooth change in the output of the internal-combustion engine can be reduced at the time of the switching among the intake cams. However, in a situation wherein the intake cam on the high load side is operated to open/close the intake valve in the vicinity of a load where the intake cam switching is carried out, the intake valve will open at a relatively large opening degree in an exhaust stroke. Accordingly, the amount of an exhaust gas charged together with air into the combustion chamber during an intake stroke following the exhaust stroke tends to suddenly change. This may cause a misfire or the like, resulting in deteriorated output performance of the internal-combustion engine.
An internal-combustion engine used as a motive power source of a vehicle or the like is generally required to operate under a variety of environmental conditions, so that the internal-combustion engine is desired to be capable of achieving a successful startup at a variety of ambient temperatures and atmospheric pressures.
In this case, various experiments and studies carried out by the inventors of the present application have revealed that whichever intake cam selected for driving the intake valve at the time of starting up the internal-combustion engine influences the startability of the internal-combustion engine. For this reason, when starting up the internal-combustion engine, it is desired to properly select the intake cam used for driving the intake valve at the startup of the internal-combustion engine.